1. Field of the Invention
This invention relates generally to measurement and data acquisition systems and, more particularly, to filter design.
2. Description of the Related Art
Scientists and engineers often use measurement systems to perform a variety of functions, including measurement of a physical phenomena or unit under test (UUT), test and analysis of physical phenomena, process monitoring and control, control of mechanical or electrical machinery, data logging, laboratory research, and analytical chemistry, to name a few examples.
A typical measurement system comprises a computer system with a measurement device or measurement hardware. The measurement device may be a computer-based instrument, a data acquisition device or board, a programmable logic device (PLD), an actuator, or other type of device for acquiring or generating data. The measurement device may be a card or board plugged into one of the I/O slots of the computer system, or a card or board plugged into a chassis, or an external device. For example, in a common measurement system configuration, the measurement hardware is coupled to the computer system through a PCI bus, PXI (PCI extensions for Instrumentation) bus, a GPIB (General-Purpose Interface Bus), a VXI (VME extensions for Instrumentation) bus, a serial port, parallel port, or Ethernet port of the computer system. Optionally, the measurement system includes signal conditioning devices which receive field signals and condition the signals to be acquired.
A measurement system may typically include transducers, sensors, or other detecting means for providing “field” electrical signals representing a process, physical phenomena, equipment being monitored or measured, etc. The field signals are provided to the measurement hardware. In addition, a measurement system may also typically include actuators for generating output signals for stimulating a UUT.
Measurement systems, which may also be generally referred to as data acquisition systems, may include the process of converting a physical phenomenon (such as temperature or pressure) into an electrical signal and measuring the signal in order to extract information. PC-based measurement and data acquisition (DAQ) systems and plug-in boards are used in a wide range of applications in the laboratory, in the field, and on the manufacturing plant floor, among others.
Typically, in a measurement or data acquisition process, analog signals are received by a digitizer, which may reside in a DAQ device or instrumentation device. The analog signals may be received from a sensor, converted to digital data (possibly after being conditioned) by an Analog-to-Digital Converter (ADC), and transmitted to a computer system for storage and/or analysis. Then, the computer system may generate digital signals that are provided to one or more digital to analog converters (DACs) in the DAQ device. The DACs may convert the digital signal to an output analog signal that is used, e.g., to stimulate a UUT.
Multifunction DAQ devices typically include digital I/O capabilities in addition to the analog capabilities described above. Digital I/O applications may include monitoring and control applications, video testing, chip verification, and pattern recognition, among others. DAQ devices may include one or more general-purpose, bidirectional digital I/O lines to transmit and received digital signals to implement one or more digital I/O applications.
Generally, signals that are being measured using a DAQ system are first routed from a particular channel via a multiplexer. The signals then enter an instrumentation amplifier, typically a programmable gain instrumentation amplifier (PGIA). The PGIA typically applies a specified amount of gain to an input signal, which raises the signal to a higher level and ensures proper A/D conversion. The amplifier may also convert differential input signals applied to the DAQ board to a single-ended output so that the ADC can correctly digitize the data. Rather than being routed directly to an ADC, the output of a PGIA is typically sent to a filter, or filter bank, and the filtered output is then provided to the ADC for conversion. The ADC may then sample and hold the signal until the signal is digitized and placed into a FIFO buffer on the board. In the FIFO, the digitized signal is ready to be transferred from the board to computer memory via the PC bus for further processing.
Filtering of the output of the PGIA is generally performed to reduce noise, since noise typically results in measurement uncertainty. Since many of the new generation ADCs feature differential inputs, differential filtering is preferred. One way to obtain differential filtering if the input signal is fully differential is to use a pair of single-ended filters. This method typically suffers from additional noise from two op-amps being added to the signal. There may also be a need for common-mode level shifting, which generally requires additional circuitry, thus further increasing the potential for noise. In case of single-ended input signals the need arises for a single-ended-to-differential converter, either following a single-ended filter or preceding a pair of single-ended filters. In either case, the components of the single-ended-to-differential converter typically add more noise to the signal, partially defeating the original purpose of the filter itself.
Differential active filters have typically been built around differential op-amps. In one set of applications, feedback capacitors are added in parallel with the feedback resistors. This typically provides filtering, but only of first order, which is insufficient for many applications. To achieve higher order filtering, a differential version of the “multi-feedback” filter has been employed in an array of applications. This method however includes adding extra noise-generating resistors to the circuit in order to obtain the second filter pole. Such resistors would be unnecessary in a single-pole filter. A differential passive (LCR) filter may provide an alternative solution, however the nonlinearity of the inductors that may be comprised in a passive filter could result in additional problems, as well as potentially increasing the filter's susceptibility to magnetic noise coupling. In addition, a passive filter offers no provision for common-mode level shifting.
Therefore, a differential filter that can accomplish single-ended to differential conversion, common-mode level shifting, and second-order filtering with a minimum of noise-generating components is highly desirable.
It is sometimes desirable to have a choice of multiple filter cutoff frequencies. For example, it may be desirable to be able to choose a wide bandwidth for fastest settling or a low bandwidth for minimum noise. In such instances, and others as well, there may be multiple stages or instances of filtering, where some of the stages or instances are unused. To avoid undesired coupling between stages in such cases, the unused stages of filters should be disabled. It is therefore desirable to have a means to disable the output of a differential filter.
It is also sometimes the case that a filter does not settle as well as would be expected due to the presence of dielectric absorption (DA) in the capacitors used in the filter. It is therefore desirable to provide a means by which the DA-induced errors of a filter stage can be compensated.
Other corresponding issues related to the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.